During a read process of a data storage device, a read head is typically passed over a data record recorded on a storage media in order to convert the recorded data into an analog signal. For example, in a magnetic tape drive, a magneto-resistive read head is passed over a data record that has been previously written as flux reversals on a magnetic tape. As the head is passed over the tape, the head converts the flux reversals into an electrical analog signal that represents the data originally stored on the magnetic tape.
An analog to digital converter (“ADC”) periodically samples the analog signal and converts the sampled analog signal to a digital input signal and creates a digital waveform. The amplitude of the digital waveform is processed to form binary values. The data storage device, such as a magnetic media storage device, processes the digital waveform to reconstruct the data that was originally written to the tape and passes the data to a host device.
Because of noise in the read channel, a binary value corresponding to one or more digital input signals may be indeterminate. For example, one or more of the digital input signals may have a lower amplitude, which is corrupted by noise, increasing the difficulty of reconstructing the data and determining the binary value.
Storage devices such as magnetic tape drives frequently use a maximum-likelihood detector to reconstruct data from the digital waveform. Such a detector employs a plurality of states organized as a logical trellis. The trellis specifies allowable subsequent states for each state. For example, the trellis may specify that the detector may change from a first state to a second or a third state, but not to a fourth state. The detector thus changes from the first state to the second state or to the third state depending on a plurality of branch metrics. A branch metric for the current state is a function of branch metrics for previous states and of a current digital input signal. Thus, path metrics maintain a record of the detector's progression through previous states. The detector begins in an initial state and proceeds to change from state to state in response to the branch metrics, identifying the binary value for one or more digital input signals using information about previous and subsequent digital input values from the path metrics. As a result, the detector is better able to reconstruct data from digital input values.